This invention relates to the fields of digital integrated circuit testers and bipolar integrated circuitry, and more particularly to the design of a pin driver circuit for such testers that is suitable for implementation in a monolithic bi-polar n-p-n cell and which has programmable high and low outputs, symmetrical rise and fall times, low output impedance, tri-stateability, and protection against static discharges and short circuiting.
Previously it has been thought that the attainment of a low impedance, completely controllable pin driver output with symmetrical rise and fall times required the use of both n-p-n and p-n-p parts. The need for both n-p-n and p-n-p circuit elements dictated the use of discrete components, or the use of hybrids, or semiconductors with both n-p-n and p-n-p circuit elements wherein the properties of the p-n-p elements were less effective than those of their n-p-n counterparts
The all n-p-n circuit that is most frequently used is a pull-up emitter follower in combination with a pull-down current source. However, this circuit lacks rise and fall symmetry, because the fall time arising from the use of the current source varies with the amplitude of the signal according to a linear function, while the emitter follower output varies according to an exponential function with a typical time constant for all amplitude values. If the application is not sensitive to this difference and only one output level is required, this solution is adequate. However, for the pin driver circuitry of an integrated circuit tester with maximum flexibility, it is highly desirable to be able to vary the voltage levels of the output while achieving symmetry between the rising and falling edges of the output waveform.